This invention relates to new and novel construction of thermal storage apparatus used with heat recovery regenerators so that exhaust contaminant condensed in heat transfer passages during industrial heat treating processes can be removed.
Heat recovery regenerators recover exhaust heat by passing exhaust gases through heat exchange elements carried in thermal storage units thus causing the element temperature to increase and the exhaust temperature to decrease. Heat is stored in the heat exchange elements in the thermal storage units. After a predetermined period of time the exhaust flow stops and clean replacement air for the oven is caused to flow through the same passages of the heat exchange elements but in the opposite direction. This flow of cool replacement air from the atmosphere causes the heat exchange element temperature to decrease and replacement air temperature to increase. After another predetermined period of time replacement air flow stops and the cycle just described is repeated over and over again.
In practice, a set of two thermal storage units is used such that one unit is passing exhaust while the other passes replacement air. This feature permits exhaust to continuously flow from the oven while at the same time permits replacement air to continuously flow into the oven. Heat recovered from exhaust gases has been returned to the oven as hot replacement air thus reducing the amount of fuel needed to generate heat for the oven's industrial processes.
Since exhaust and replacement air flow in opposite directions through the set of heat exchange elements, hot exhaust always enters the thermal storage unit from one end and cold replacement air always enters from the opposite end. For efficient heat exchange operation, the recycle time is kept short. If not cycled frequently a pronounced axial temperature gradient does not develop and high recovery efficiency is not developed. Therefore, the exhaust end of thermal storage is always hot and the replacement air end of thermal storage is always cold.
U.S. Pat. No. 3,712,597 discloses a series of bayonet type metallic recuperative heat exchangers utilized in combination with a continuous rotary regenerated air preheater in a glass manufacturing oven. The plugging and cleaning problems created by lint, oils and resins exhausted from industrial dryers are a major deterent to the use of heat exchangers.
In applicant's U.S. Pat. No. 4,337,585 entitled HEAT RECOVERY AND AIR PREHEATING APPARATUS FOR TEXTILE DRYER OVENS, exhaust heat recovery and replacement air preheating apparatus is disclosed which includes a set of heat recovery and storage units installed in the walls of the dryer oven so that hot air exhausted from the dryer passes through one of the heat recovery and storage units. Cool replacement air simultaneously enters the dryer through the other of the heat recovery and storage units.
Typically, the prior arrangements have utilized heat storage elements arranged serially in the thermal storage units which are identical in construction and function which is primarily to absorb and store heat. Reverse flows through the storage unit removes lint and other solid contaminant particulate matter from the elements which act secondarily as filters.
The exhaust carries two kinds of contamination which in combination tend to plug heat exchangers. Lint made of fine fiber or dust will plug exchangers if the flow is continuous in direction. With this invention the reversing flow direction back flushes line out of the heat storage before it passes the hot end so it does not plug or move through with smoke to the cool end. The second contamination in the exhaust is smoke consisting of volatized oil and resin. Volatized oil and resin will condense when it contacts cool surfaces. It passes as gas through the hot storage end and condenses at the cold end but the lint has been separated out so buildup on surfaces is minimal. In U.S. Pat. No. 4,337,585, removal of the stationary storage elements is permitted at the cold end for simple periodic cleaning without interrupting dryer operation. Thus, condensed oil and resin buildup can be held at operational levels. The heat of exhaust is effectively prevented from reaching the ambient environment and the ducts remain relatively cool eliminating the need for costly insulation.
However, it has been found that through inadvertence and neglect, the heat exchange elements on the cold end of each thermal storage unit are not removed and cleaned regularly. If the elements on the cold end are not cleaned regularly, flow passages will eventually plug rendering the thermal units inoperative and will present a fire hazard. The cold end thus requires more frequent cleaning and it is often necessary to climb ten to twenty feet above floor level to reach the thermal storage unit on top of a large industrial oven. To accomplish practical heat recovery operation, thermal storage must be structured to accommodate cleaning condensible contaminants in a convenient and economical manner.
It has been found according to the present invention that the function and hence structure of the heat exchange elements at the hot and cold ends should be different. With respect to lint removal at the hot end, the heat storage elements function like the functioning of a filter to remove lint and other solid matter from the exhaust flow. With respect to removal of condensing volatiles at the cold end, the heat storage elements function like the functioning of a condenser and precipitate out resins and oils in the exhaust gas flow. The lint and other solid matter is cleaned from the hot end heat storage filter elements by reverse flow while the condensated resins and oils are removed by providing disposable condensor elements and/or draining the condensate from the cold end of the unit.
Accordingly, an important object of the present invention is to provide a heat recovery and storage apparatus for preheating replacement air in industrial ovens wherein the buildup of condensate in heat exchange elements on the cold end of a heat transfer unit and the problems attendant to the cleaning of the elements are reduced.
Another important object of this invention is to provide thermal storage units with heat storage elements of different scale and geometry in flow passages near the cold end to accept selective buildup of condensing contaminants without inhibiting air flow.
Another object of this invention is to provide thermal storage units having heat exchange elements fabricated from different materials or coated materials to accommodate high temperatures near the hot end of the storage units and to more easily accommodate cleaning contaminants condensed from exhaust near the cold end.
Still another object of the invention is to provide industrial thermal storage structure with heat exchange elements near the cold end that are different from those near the hot end in that they are lighter in construction to be more easily handled and carried for removal and/or cleaning.